Light trapping within silicon substrates for solar cells has received considerable attention in recent years. The highest efficiency silicon solar cell reported by Wang et alia, Appl. Phys. Lett. 57 (6), p. 602, has been measured to have an efficiency of 24.2% under standard test conditions. Such high performance silicon cells make extensive use of geometrical structures chosen to cause light to be multiply totally internally reflected giving substantial increases for the pathlength the light traverses before escaping from the silicon. Due to the large difference between the refractive index of silicon and that of air, such light trapping schemes can be designed relatively simply while being quite effective. For a transparent medium such as glass, however, where the refractive index is only about 1.5 or less, it is relatively more difficult to totally internally reflect virtually all light within the transparent medium that strikes the air interface. This is because the escape angle for glass and similar refractive index media is approximately 45.degree., with the consequence that all light within the glass that strikes the glass/air interface within 45.degree. to the normal will escape to the air.
In the area of concentrator solar cells, various schemes and designs for the optics have been proposed for the purpose of steering the light onto the solar cell surface. One such scheme is that proposed by Mills and Giutronich, Solar Energy 21 (1978), pp. 423-430, where the front and rear surfaces of a glass or equivalent structure are at an appropriate angle to each other to ensure that light reflected from the rear surface strikes the glass/air interface at the top surface outside the escape angle and is therefore totally internally reflected. In this scheme, a solar cell is mounted perpendicular to the top surface and at the end of the wedge-shaped structure so as to receive the light that has been trapped within the structure. This scheme is shown in FIG. 1 which is a schematic cross section side view of a device 100 for directing light in two dimensions. In FIG. 1 a beam of incoming light 101 passes through top surface 102 of triangular glass structure 103 to point 104A, is reflected at point 104A by reflector 104 to point 102A, totally internally reflected at point 102A by surface 102 to point 104B and is reflected at point 104B by reflector 104 to point 105A where it is absorbed by top surface 105 of solar cell 106. One severe limitation of this scheme for many applications is the large value of .theta. required to ensure light reflected from reflector 104 is totally internally reflected when subsequently striking the glass/air interface at surface 102. This large value of .theta. results in dimension A having to be relatively large in comparison to dimension B unless the acceptance angle for incoming light 101 is greatly reduced. Furthermore, in this scheme, no light falls directly onto cell 106, only reflected light.